Removing the wheels would allow placing them in a shed or garage, which seems like a very effective way to protect them from UV light
. I don't bother, but then I'm due for new tires anyway and have no need to protect the old ones; also, my car tires don't fall
apart over a year or two sitting outside (holding up the car), so it doesn't seem like a big concern to me.
Removing the wheels suggests that the trailer would not be supported on the suspension, but instead on the frame. I think this is fine, as long as the supporting jacks hold the frame reasonably close to the axle
(and presumably behind the axle
, rather than in front, since the other point of support will be at the tongue).
I wouldn't worry about the exposed hubs (either plain hubs or brake drums): I don't think the wheels provide meaningful protection for them anyway. I would rather leave them open to get wet and dry with the weather than to enclose them in plastic and risk trapping water.